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Emmanuel Ax (at left) is more than a known quantity to most piano enthusiasts; he's something like an old friend, always delivering reliable pleasure, but also perhaps limited by that very familiarity - we almost know him too well. Not that he's technically limited; indeed, his brilliant Celebrity Series concert at Jordan Hall on Sunday demonstrated (once again) that he can do practically anything at the keyboard. What one faces with Mr. Ax is an issue that affects many a virtuoso (and which it's almost crass to carp at): a settled sensibility whose attempts at re-invention never quite convince.

Ax's signature has always been a kind of intelligent intimacy attended by classic poise and proportion, not to mention a superb command of musical surface - he can draw either a dazzling shimmer or a plush sense of depth from the keyboard seemingly at will. A deeper interpretive profile, however, seems to elude him, as well as that sense of mystery that profound musical architecture can exude.

Thus during his Celebrity Series program Ax was most compelling in his selections from chamber music (Schumann's Papillons) or early, "classical" Beethoven (Piano Sonata No. 2). This rarely-played sonata proved unexpectedly pleasing and mature: the first two movements were intriguingly complex, while Ax brought off the lyricism of the ensuing scherzo and rondo with serene aplomb. Likewise the familiar Papillons was generally a sparkling delight.

With Schumann's Humoreske, however, Ax seemed to meander, and no wonder; the piece is famously free-form, with a "secret theme" buried in the score that is never actually articulated by the pianist. Evoking this quasi-mystical "key" would be a challenge for any performer, and alas, Ax didn't bring it off, although the separate, contrasting movements were often appealing in and of themselves. Perhaps a greater problem haunts the heroic Waldstein sonata, Ax's final selection. Beethoven's warhorse cries out for a personal stamp of some kind, but Ax seemed to settle for variations on his trademark rubato: accelerating through the piece's famous chord-tremors with a speed that verged on the superhuman (and which it seemed even he couldn't quite control), Ax was forced to hit the brakes whenever he wanted to actually phrase the score. It was a thrilling, but ultimately superficial performance - still, the crowd loved it, and in his encore Ax threw them a bouquet in return: a charming, unforced reading of Chopin's Waltz in A minor. Unaffected, direct, and superb, it was a lovely coda, and perhaps the finest performance of the evening.